The present invention relates to a compact wide-angle lens for use in digital still cameras, video cameras, and other devices that are equipped with an imaging element having a high pixel count such as a CCD.
A lens in a security camera is a conventional wide-angle lens that captures moving images. The conventional wide-angle lens in a security camera does not require high-quality optical characteristics since the imaging element has a relatively low pixel count.
A conventional digital still camera, video camera, and other related devices are used to capture images and do not require high-quality optical characteristics. Additionally, a thin design is desirable for these devices.
Imaging element technology has rapidly advanced in recent years. The advances have led to imaging elements with higher pixel densities and pixel counts and a need for lenses with high-quality optical characteristics.
A recent application of imaging element technology involves the use of a digital still camera to take still images. The still images are transferred to a personal computer to be able to apply various forms of processing. Imaging elements with high pixel densities and high pixel counts are used in digital still cameras to work well with high-resolution displays.
However, few high-quality lenses can handle imaging elements with high pixel densities and high pixel counts. Therefore, there is a demand for lenses that are compact, thin, compatible with high-resolution displays, and the like. A thinner design is particularly important since the size of an imaging element is ⅓ inch or less and, more recently, {fraction (1/2.7)} inch.
Imaging elements such as CCD""s have a micro-lens attached to the surface of the imaging element. The micro-lens uses incident light efficiently. However, eclipsing takes place if the angle of the incident ray is too large, and the light does not reach the imaging element.
The present invention overcomes the problems described above and provides a wide-angle lens with high-quality optical characteristics that eliminates eclipsing and the like. The wide-angle lens provides a thin, compact, and light-weight design and is inexpensive. The wide-angle lens is suited for imaging elements with high pixel counts of 2 million-3 million.
A wide-angle lens of the present invention includes, from an object side to an image plane side: a first lens group and a second lens group. The first lens group includes a first lens with a negative refractive power and a second lens with a positive refractive power. The second lens group includes a third lens with a negative refractive power, a fourth lens with a positive refractive power, and a fifth lens with positive refractive power. The fourth lens is bonded to the third lens, and the fifth lens is formed with convex surfaces oriented to the object side and the image plane side. At least one of the convex surfaces of the fifth lens is an aspherical surface.
The wide-angle lens satisfies the following conditions (1), (2), (3), and (4):
(1) 0.7|R6| less than |R8| less than 1.3|R6|,
(2) v1 greater than v2, v3 less than v4, v5 greater than 50,
(3) |f1| greater than 2 f2, and
(4) 2.5 f22 greater than f21 greater than f22,
where R6 is the curvature radius on an object-side surface of the third lens; R8 is the curvature radius on an image plane side of the fourth lens; vi is the Abbe number of i-th lens (i=1-5); f1 is the composite focal length of the first lens group; f2 is the composite focal length of the second lens group; f21 is the composite focal lengths of the third and the fourth lenses in the second lens group; and f22 is the focal length of the fifth lens in the second lens group.
The total lens length of the wide-angle lens (not including the back focus) is no more than 12 mm. The back focus is at least 7 mm to allow for the placement of low-pass filters and the like. Additionally, the exit pupil position is at least |20 mm| to prevent eclipsing and maintains brightness with a F number of approximately 2.8.
Alternatively, both of the convex surfaces S9, S10 of the fifth lens of the wide-angle lens can be aspherical surfaces. This wide-angle lens satisfies the following conditions (1), (2), (3), and (4):
(1) 0.7|R6| less than |R8| less than 1.3|R6|,
(2) v1 greater than v2, v3 less than v4, v5 greater than 50,
(3) f1 greater than 4 f2, and
(4) 2.5 f22 greater than f21 greater than f22,
where R6 is the curvature radius on an object-side surface of the third lens; R8 is the curvature radius on an image plane side of the fourth lens; vi is the Abbe number of i-th lens (i=1-5); f1 is the composite focal length of the first lens group; f2 is the composite focal length of the second lens group; f21 is the composite focal lengths of the third and the fourth lenses in the second lens group; and f22 is the focal length of the fifth lens in the second lens group.
A wide-angle lens having the structure described above is provided with the total lens length (not including the back focus) being 10 mm or less, the back focus being at least 7 mm, and the exit pupil position being at least |20 mm|. The wide-angle lens is compact and thin and has good optical characteristics that are suited for high-density imaging elements with high pixel counts.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.